1. Field of the Invention
The present invention generally relates to ISDN (Integrated Services Digital Network, and more particularly to a method of controlling ISDN terminal equipments, such as audio/visual terminal equipments or audio terminal equipments using the ISDN as a transmission path, connected to a passive bus of an ISDN basic interface.
2. Description of the Prior Art
The types of data transmittable via an ISDN being built at present are audio data, dynamic image data, still image data and general-purpose data. Recently, multimedia terminals capable of transmitting a plurality types of data have been in practical use.
Now, the CCITT is preparing for the provision of recommendations regarding audio/visual terminals capable of handling audio data and dynamic image data, such as video phone and television conference systems. In Japan, the TTC has made the national recommendations based on the recommendations of the CCITT.
Basically, the audio/visual terminal uses one or two information channels (B channels) having a bit rate of 64 Kbps, and dynamically uses secured B channels in order to transfer audio data having bit rates of 16, 48, 56 and 64 Kbps and dynamic image data having bit rates of 46.2, 62.4, 68.8, 76.8, 102.4 and 108.8 Kbps or other bit rates. The audio data and the dynamic image data can be transmitted so that they are combined with each other, or separately from each other. It will be noted that encoding of audio and dynamic image data using a transmission capacity of 128 Kbps can provide quality higher than that using a transmission capacity of 64 Kbps.
According to the ISDN basic interface, two information channels for use in data transmission and one signal channel (D channel; 16 Kbps) for use in transfer of signals for call setup and low-bit rate packet data communications are multiplexed with each other. Connection points (S/T points) of the ISDN basic interface can be connected to a passive bus. The passive bus has a capacity such that 32 sockets (consents) can be connected thereto, and eight terminal equipments which are maintained in the transmittable state can be connected to the passive bus at most. Thus, by using the passive bus, it is possible to separately and independently use audio/visual terminals (video phones, for example) and other ISDN terminals, such as telephone terminals.
However, the above separate and independent use of audio/visual terminals and other ISDN terminals connected to a single passive bus has the following disadvantages. When data is transferred between video phones in a state where a telephone set connected to a passive bus to which the other party video phone is connected is busy, one information channel is available between the video phones. In the above case, another video phone can be used for communication. During this communication, audio information is encoded in a band of 3.4 kHz and a bit rate of 16 Kbps is allotted thereto. Further, dynamic image data is encoded in accordance with a QCIF (image format described in the TTC Recommendation JT-H.242). Regarding the dynamic image data, the MPI of a screen interval is set to approximately 29.97/2, and a bit rate of 76.8 Kbps is allotted, so that the sum total of these rates becomes equal to 124.8 Kbps. In the above-mentioned manner, a high-quality communication mode can be obtained. However, it is impossible to identify a timing at which the second information channel is ensured unless the calling telephone terminal or the called telephone terminal can detect the end of speech. In this case, it is possible to shift to the high-quality communication mode.
According to the existing recommendations, there is no way except for a process by trial and error in order to determine whether or not the second information channel can be ensured. In the process by trial and error, an additional call setup operation must be carried out. Hence, there is a possibility that invalid calls in the ISDN may increase.